1. Field of the Invention
This invention relates generally to compressors used, for example, in air conditioning and refrigeration systems, and more particularly to a control for detecting unsatisfactory oil pressure conditions in a compressor and for protecting the compressor from damage from such conditions.
2. Brief Description of the Prior Art
The use of electronics to control and detect oil pressure for compressors is known. Conventional controls typically use time and duty cycle of good versus bad oil conditions to stop compressor operations. The prior art oil sensing controls utilized discrete electronics and a normally open oil pressure switch. Whenever a bad oil condition occurred the pressure switch would open and the discrete electronics would begin to time. If the time reached two minutes, a magnetic latching relay would be energized (i.e., the relay would latch) which in turn switched power off to the compressor. This system has the feature that if the pressure switch is disconnected, the control will time out in two minutes. The function of the control is to protect the compressor from the loss of lubrication. This is a time weighted average measurement to avoid nuisance trips (from conditions like defrost where liquid refrigerant may flood back into the compressor momentarily). To implement a time weighted average, the control must maintain the accumulated bad oil time during brief power outages (up to 2 minutes). This implies that voltage must be maintained in the control during these interruptions. When the power is interrupted, the pressure switch opens when the compressor stops (oil pressure is lost) if the pressure switch is a normally open device. This open switch limits power dissipation which in turn prolongs voltage on the control during the power interruption. This prior art system is adequate for the task but has many short comings. First, the oil pressure switch utilizes a magnetized shuttle which moves as the pressure changes and a reed switch. The magnetic shuttle moves toward the reed switch due to increasing oil pressure, which in turn causes the reed switch to close. The oil pressure switch could be made much smaller and less expensive if the pressure moved the magnetic shuttle away from the reed switch but this implies that the oil pressure switch is normally closed. Utilizing the prior art control, the maintenance of voltage during power interruptions would be impossible with a normally closed pressure switch (without expensive signal buffering). Furthermore, the prior art control does not detect if the wires to the sensor are shorted together (in fact the control will continue to allow the compressor to operate forever in this condition). The use of discrete electronics also has the disadvantage that the time to trip cannot be easily changed (i.e., the circuit board must be changed). Fault indicator lights cannot be used with the prior art control, an LED would constitute a continuous current drain on the power supply. This would lead to a loss of time during a power interruption as well. Finally, discrete electronics cannot easily filter noise signals from the sensor signals and require the use of expensive shielded cables in noisy environments.